Method for producing alkali metal niobate particles, and alkali metal niobate particles

ABSTRACT

Disclosed are a method of producing fine particulate alkali metal niobate in a liquid phase system, wherein the size and shape of the particulate alkali metal niobate can be controlled; and fine particulate alkali metal niobate having a controlled shape and size. One of specifically disclosed is a method of producing a substantially rectangular cuboid particulate alkali metal niobate represented by MNbO 3  (1), wherein M represents one element selected from alkaline metals, including specific four steps. Another one of specifically disclosed is particulate alkali metal niobate represented by the formula (1) having a substantially rectangular cuboid shape, wherein the substantially rectangular cuboid shape has a longest side and a shortest side, the length of the longest side represented by an index L max  is 0.10 to 25 μm, and the length of the shortest side represented by an index L min  is 0.050 to 15 μm.

TECHNICAL FIELD

The present invention relates to a method of producing particulatealkali metal niobate, and particulate alkali metal niobate.

BACKGROUND OF THE INVENTION

Piezoelectric ceramics have significantly contributed to downsizing andsophistication of electronic devices. In addition to applications toconventional devices such as sensors or ultrasonic transducers,piezoelectric ceramics are recently used, for example, as a raw materialof transformers for LCD backlights of personal computers or a rawmaterial of head parts of ink jet printers.

Lead-based materials such as PZT-based materials prevail nowadays assuch piezoelectric ceramic devices. However, lead-based materialscontain large amounts of harmful lead oxide, and thus, for example,environmental pollution by spilled lead oxide on disposal has been amatter of concern. Therefore, development has been strongly demanded forlead-free piezoelectric ceramic materials which can be used foralternatives to conventional lead based materials.

Recently, alkali niobate piezoelectric ceramics draw attention aslead-free ceramic materials, which exhibit relatively highpiezoelectricity. Patent Document 1, for example, proposes apiezoelectric ceramic including a solid solution mainly composed oflithium sodium niobate, together with minor components as aluminum oxideand iron oxide. Patent Document 2 proposes an improved composition for apiezoelectric ceramic, which includes potassium niobate and sodiumniobate, as main components, and copper, lithium, and tantalum, asadditional components.

As a method of producing such piezoelectric ceramics, a method called asolid phase method has been widely known. The solid phase methodtypically includes mechanically mixing or kneading plural kinds ofparticulate materials as raw materials, then pelletizing, and calciningthe obtained pellets.

In recent years, liquid phase methods of synthesizing NaNbO₃ particleshave also been studied. For example, Non-Patent Document 1 reports amethod of synthesizing NaNbO₃ particles by reacting NaOH or KOH solutionwith Nb₂O₅ particles.

Another technique has been recently reported on a method for producingparticulate KNbO₃ by once synthesizing layered K₄Nb₆O₁₇ particles, andthen heating the particles at a high temperature in a molten salt(Non-Patent Document 2).

REFERENCES [Patent Documents]

[Patent Document 1] JP 60-52098 B

[Patent Document 2] JP 2000-313664 A

[Non-Patent Document]

[Non-Patent Document 1] C. Sun et al., European Journal of InorganicChemistry, 2007, 1884

[Non-Patent Document 2] Y. Saito et al., Journal of the European CeramicSociety, 27 (2007) 4085

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, solid phase methods are disadvantageous in that nanoscaleuniform mixing of raw material particles is generally difficult becausecommonly available raw material particles often have a larger size, likeabout several millimeters to several micrometers. When raw materialparticles are calcined at a high temperature, the original crystalstructure of the raw material changes into perovskite crystal structure.Thus, it is difficult to precisely control crystallite size and grainboundaries in a solid phase method. Control of grain boundaries isespecially indispensable for enhancing properties of piezoelectricceramics because grain boundaries significantly affect some propertiessuch as piezoelectric properties or strength. Therefore, use of amaterial in which control of grain boundaries are insufficient may leadto problems such as defects of products and deterioration of properties.

Conventional liquid phase methods may cause particle agglomeration.Also, it is generally difficult to control the size and shape ofparticles in a uniform state by a conventional liquid phase method. Forexample, particles produced by the method described in Patent Document 1are given as aggregates, and are not suitable as a material to formpiezoelectric devices, for which downsizing has been recently demanded.

The method described in Patent Document 2 also requires some improvementin that control of the particle size is actually impossible or thatmulti-step synthesis is required.

In view of the current state, there has been a demand for developing amethod of producing particulate alkali metal niobate, which is suitablefor mass production, can prevent agglomeration of particles, and cancontrol the grain boundaries and particle size. Further, there has beena demand for fine particulate alkali metal niobate having a highlyuniform particle size.

Means for Solving the Problem

The present invention has an object to provide a liquid phase method ofproducing fine particulate alkali metal niobate which can control thesize and shape of the fine particulate alkali metal niobate. The presentinvention has another object to provide fine particulate alkali metalniobate having controlled shape and size, and a lead-freeniobium-containing ceramic material having high piezoelectricity.

A first aspect of the present invention relates to a method of producingparticulate alkali metal niobate represented by MNbO₃ (1)

wherein M represents one element selected from alkaline metals, theparticulate alkali metal niobate having a substantially rectangularcuboid shape,

the method including the steps of:

(a) mixing a niobium-containing solution with an alkaline solutionhaving a concentration of 0.1 to 30 mol/L, to prepare a suspension;

(b) still standing the suspension at between 80° C. and 150° C. for 12to 48 hours;

(c) performing solvothermal reaction of the suspension at between 150°C. and 300° C. for 1 to 12 hours; and

(d) separating the particulate alkali metal niobate from a reactionmixture.

In a preferred embodiment, M in the formula (1) is Na, and the alkalinesolution is NaOH.

In another preferred embodiment, M in the formula (1) is K, and thealkaline solution is KOH.

In another preferred embodiment, the niobium-containing solutionincludes:

-   -   niobium oxide and/or niobium halide;    -   a solvent selected from the group consisting of water, ethylene        glycol, and polyethylene glycol; and    -   an acid.

A second aspect of the present invention relates to particulate alkalimetal niobate represented by the formula (1):

MNbO₃   (1)

wherein M represents one element selected from alkaline metals, theparticulate alkali metal niobate having a substantially rectangularcuboid shape,

wherein the substantially rectangular cuboid shape has a longest sideand a shortest side,

the length of the longest side represented by an index L_(max) is 0.10to 25 μm, and

the length of the shortest side represented by an index L_(min) is 0.050to 15 μm.

In a preferred embodiment, the ratio of the L_(max) to the L_(min),L_(max)/L_(min), is within the range of 1 to 5.

In another preferred embodiment, M in the formula (1) is Na or K.

In yet another preferred embodiment, the particulate alkali metalniobate is prepared by the above method.

A third aspect of the present invention relates to a piezoelectricceramic material that comprises the particulate alkali metal niobate.

Effect of the Invention

According to the production method of the present invention, particulatealkali metal niobate, preferably fine particulate NaNbO₃ or KNbO₃ can besynthesized in a large scale while controlling the size and shape.Resulting particles have a unique shape of substantially rectangularcuboid, more preferably substantially cubic, and the size and shape arewell controlled. The method of the present invention is advantageousbecause the method gives practically favorable sub-micron to severalmicrometer particles in a manner suitable for mass production.

In addition, ceramic materials obtained by pelletizing the niobateparticles and calcining the resultant pellet is more advantageous thanniobium-based piezoelectric ceramic materials obtained by conventionalsolid phase methods in the following points:

1. Low-temperature calcination is practicable;

2. Excellent piezoelectric properties will be exhibited;

3. Densification of ceramic materials are easily achievable; and

4. Slurry preparation prior to production of layered articles is easy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram for explaining of the particulatealkali metal niobate of the present invention having a substantiallyrectangular cuboid shape.

FIG. 2 shows a SEM image of particulate NaNbO₃ synthesized in Example 1.

FIG. 3 shows XRD (X-ray diffraction) pattern of the particulate NaNbO₃synthesized in Example 1.

FIG. 4 shows a SEM image of particulate KNbO₃ synthesized in Example 2.

FIG. 5 shows a SEM image of particulate KNbO₃ synthesized in Example 2(Magnified image of the FIG. 4).

FIG. 6 shows a SEM image of fine particulate KNbO₃ synthesized inExample 2 (temperature at secondary heating: 150° C.).

FIG. 7 shows a SEM image of particulate KNbO₃ synthesized in Example 4(starting material: niobium oxide).

FIG. 8 shows a SEM image of particulate NaNbO₃ prepared in Example 5(heated at 100° C.).

FIG. 9 shows a SEM image of particulate NaNbO₃ prepared in ComparativeExample 1 (not heated at 100° C.).

FIG. 10 shows a SEM image of particulate KNbO₃ synthesized in Example 6(large scale synthesis).

FIG. 11 shows a SEM image of particulate KNbO₃ synthesized in Example 6(Magnified image of the FIG. 10).

FIG. 12 shows an XRD pattern of particulate KNbO₃ synthesized in Example6 (large scale synthesis).

MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail below.

<Method of Producing Particulate Alkali Metal Niobate>

As mentioned above, a first aspect of the present invention relates to amethod of producing particulate alkali metal niobate represented by theformula:

MNbO₃   (1)

wherein M represents one element selected from alkaline metals, theparticulate alkali metal niobate having a substantially rectangularcuboid shape,

the method including the steps of:

(a) mixing a niobium-containing solution with an alkaline solutionhaving a concentration of 0.1 to 30 mol/L, to prepare a suspension;

(b) still standing the suspension at between 80° C. and 150° C. for 12to 48 hours;

(c) performing solvothermal reaction of the suspension at between 150°C. and 300° C. for 1 to 12 hours; and

(d) separating the particulate alkali metal niobate from a reactionmixture.

In the formula (1), M is an alkaline metal. Specifically, M is anelement selected from the group consisting of lithium (Li), sodium (Na),potassium (K), rubidium (Rb), and cesium (Cs). Preferably, M is Li, Na,or K, and more preferably Na or K. In particular, M is preferably Kbecause obtained particles are small in size, and have a highly uniformclose-to-cube shape.

In the following, each step is described.

The step (a) is for preparing a suspension by mixing aniobium-containing solution as a niobium source with ahigh-concentration alkaline solution.

The method to prepare a niobium-containing solution is not particularlylimited. For example, such a solution can be prepared by dissolving aniobium compound in an acidic liquid solvent. Preferably, such a niobiumcompound may be, but not limited to, at least one of niobium oxide andniobium halides. Examples of the niobium halides include niobiumfluoride, niobium chloride, niobium bromide, and niobium iodide. In viewof handling ability and reactivity, niobium chloride is preferable amongthe niobium halide. Niobium compounds maybe used alone or in combinationof two or more of these.

Solvents to be contained in the above acidic liquid solvent are notparticularly limited. Examples thereof include water, alcohols such asmethyl alcohol and ethyl alcohol, and polyols such as ethylene glycol(EG), glycerol, and polyethylene glycol (PEG). Of these, water, ethyleneglycol, and polyethylene glycol, as well as a mixture of these, arepreferable in view of relatively high boiling points and applicabilityto solvothermal reaction. Water is particularly preferable.

The acid to be contained in the above acidic liquid solvent is notparticularly limited. Examples thereof include inorganic acids such ashydrochloric acid, sulfuric acid, and nitric acid, and organic acidssuch as trifluoroacetic acid. Of these, hydrochloric acid and nitricacid are preferable in that they are easily removable after thereaction. Hydrochloric acid is particularly preferable.

Then, the alkaline solution to be used in the step (a) is described.

In the present invention, the alkaline solution is not particularlylimited as long as it satisfies required high concentration. However,such an alkaline metal normally a source of the “M” in the particulatealkali metal niobate MNbO₃, and thus, specifically, the alkaline metalis preferably an alkali metal hydroxide represented by the followingformula (2):

MOH   (2)

wherein M is the same as defined in formula (1). NaOH or KOH isparticularly preferred among them.

The solvent contained in the alkaline solution is not particularlylimited, and may be water, alcohol, diol, triol, or acetone. Of these,water is preferred.

The alkaline solution to be used in the present invention has such ahigh concentration as 0.1 to 30 mol/L. The concentration is equivalentto that of a very-high-concentration alkaline solution having a pH ofabout 13 or higher. That is, assuming that the degree of ionization of astrong base (such as NaOH and KOH) is 1 irrespective of theconcentration of the alkaline solution, the pH of a “0.1 mol/L” alkalinesolution corresponds to 13, as follows:

[OH⁻]=1.0×10⁻¹ mol/L,

[H⁺] [OH⁻]=1.0×10⁻¹⁴,

and thus,

[H⁺]=1.0×10⁻¹³,

pH=−log [H⁺]=13

An alkaline solution having a concentration of less than 0.1 mol/L isundesirable because particles may not grow sufficiently, and thusparticles with a desired size and shape may not be produced. Incontrast, if the concentration exceeds 30 mol/L, an alkaline solutionusually reaches saturation. Thus, the upper limit of the concentrationof the alkaline solution herein actually means a saturationconcentration of the alkaline solution, and this upper limit may varydepending on the nature of the alkali. The lower limit of theconcentration of the alkaline solution is preferably 1 mol/L, and morepreferably 2 mol/L. The alkaline solution used herein is a fairly highconcentration solution. Therefore, much attention is required to handlethe solution. The reaction vessel for step (a) is preferably, but is notlimited to, a corrosion-resistant vessel made of, for example,Teflon(™).

The niobium-containing solution and the alkali solution preparedseparately in the above-mentioned manner are mixed together to prepare asuspension. The way of addition of the solutions is not particularlylimited. For example, the niobium-containing solution may be added tothe alkaline solution, or the alkaline solution may be added to theniobium-containing solution. In view of safety, it is preferable toslowly add a niobium-containing solution dropwise into the alkalinesolution over a sufficient period of time. Temperature and pressureduring the mixing are not particularly limited. Usually, the mixing maybe carried out at an ordinary temperature (15° C. to 30° C.) under anordinary pressure (about 1 atm).

Next, the step (b) is described.

The step (b) is a step of heating the suspension at a relatively lowtemperature over a long period of time. The method of the presentinvention is characterized by including two steps, namely, a step ofheating the suspension at a relatively low temperature over a longperiod of time, and a step of performing solvothermal reaction at a hightemperature for a short period of time. If the step (b) is omitted,aggregates are normally generated, so the particle size cannot besufficiently controlled. Also, if the step (b) is omitted, particleshaving a substantially rectangular cuboid shape cannot be produced,which impairs a characteristic of the present invention.

In the step (b), the suspension is heated to a temperature of between80° C. to 150° C. Keeping this temperature constant for a certain periodof time gives a uniform precursor, and encourages the particles to growinto a substantially rectangular cuboid shape. The temperature onheating is preferably 80° C. to 120° C., more preferably 90° C. to 110°C., and still more preferably the boiling point of a solvent. If wateris used as the solvent, the suspension is preferably heated to 100° C.

The step (b) is characterized by allowing the suspension to still standat a specific temperature for 12 to 48 hours. Such still standing stepfor awhile helps to produce a uniform precursor solution or suspensionsuitable for particle growth, and promotes growth of particle into asubstantially rectangular cuboid shape. If the period for still standingis too short, growth into uniform precursors maybe insufficient. Incontrast, if the period is too long, the effects may be saturated andthe step is not advantageous from an economical viewpoint. Therefore, anappropriate period of still standing is 12 to 48 hours. The period ofstill standing is preferably 15 to 36 hours, more preferably 18 to 30hours, and still more preferably 20 to 26 hours.

While the pressure during the step (b) is not particularly limited, thestep is usually performed under an ordinary pressure (about 1 atm(=about 0.10 MPa)).

Next, the step (c) is described.

The step (c) is a step of subjecting the suspension which was heated ata relatively low temperature in the step (b) to solvothermal reaction ata high temperature.

The solvothermal reaction is a reaction performed under a moderate tohigh degree of pressure (normally 1 to 10,000 atm (=0.10 to 1,000 MPa))and temperature (normally 100° C. to 1,000° C.). When water is used as asolvent, the solvothermal reaction is specially referred to as“hydrothermal reaction”. By performing this process, crystal structuresand particle shape can be controlled.

In the present invention, the solvothermal reaction is performed at atemperature of between 150° C. and 300° C. While not particularlylimited, the temperature is preferably 150° C. to 250° C.

The period of time for the solvothermal reaction is not particularlylimited, and is usually 1 to 72 hours, preferably 1 to 8 hours, and morepreferably 2 to 5 hours.

The pressure during the solvothermal reaction is not particularlylimited, and is usually 0.10 to 4.0 MPa.

Next, the step (d) is described.

The step (d) is a step to separate the particulate alkali metal niobatefrom the reaction product of the solvothermal reaction.

The method to separate the particulate alkali metal niobate is notparticularly limited. Desired particulate alkali metal niobate can beseparated through normal processes such as filtration, washing, anddrying. The number of times of washing, solvents to be used for washing,and other conditions are not particularly limited, and may beappropriately selected.

<Particulate Alkali Metal Niobate>

Particulate alkali metal niobate, which is a second aspect of thepresent invention, is described. The particulate alkali metal niobate ofthe invention is particulate alkali metal niobate represented by thefollowing formula (1):

MNbO₃   (1)

wherein M represents one element selected from alkaline metals.

The alkali metal niobate have a substantially rectangular cuboid shape.The substantially rectangular cuboid shape has a longest side and ashortest side. Assuming that the length of the longest side isrepresented by an index L_(max), and the length of the shortest side isrepresented by an index L_(min), the L_(max) is 0.10 to 25 μm, and theL_(min) is 0.050 to 15 μm.

The M in the above formula (1) is specifically one element selected fromlithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium(Cs). Li, Na, or K is preferable, and Na or K is more preferable. The Mis particularly preferably K because the particles of KNbO₃ have auniform size and shape.

The particulate alkali metal niobate of the present invention ischaracterized in that the shape of the particles is rectangular cuboid,and their size and shape are highly uniform. The shape of theparticulate alkali metal niobate can be controlled in a simple chemicalprocess, and no physical grinding process is necessary. Furthermore, theshape of resulting particles is not a spherical, which is common forparticles, but a unique, substantially rectangular cuboid shape. Suchfeatures are not expectable from the conventional knowledge.

The particulate alkali metal niobate of the present invention is in afine, substantially rectangular cuboid shape. Particulate alkali metalniobate obtained by a conventional method is generally in an aggregateform. Therefore, it has been difficult to produce micrometer-order-sizeparticles. The present invention successfully producesmicrometer-order-size particles with some advantages such as easyhandleability by controlling the shape of particulate alkali metalniobate to be in a substantially rectangular cuboid form, therebypreventing generation of aggregates.

The particles having the shape mentioned above may be packed moredensely than sphere particles or aggregated particles. Thus, gap amongparticles can be reduced on packing the particles. The particulatealkali metal niobate is thus advantageous in that a ceramic material ofthe present invention made from particulate alkali metal niobate maybecome dense.

Herein, the term “rectangular cuboid” includes an essentiallyrectangular cuboid shape and a cube shape. Further, the term“rectangular cuboid” includes a rectangular cuboid shape with partlychipped portions, or a rectangular cuboid shape with uneven surfaces,too.

The rectangular cuboid has an L_(max) the length of the longest side, of0.10 to 25 μm, and an L_(min), the length of the shortest side, of 0.050to 15 μm. A rectangular cuboid generally has 12 sides, which arerepresented as widths, depths, and heights. The particulate alkali metalniobate of the present invention shows particles well-controlled size,and has a substantially rectangular cuboid shape with the length of thelongest side of 0.10 to 25 μm and the length of the shortest side of0.050 to 15 μm.

This feature is further described with reference to the drawings.

FIG. 1 is a schematic diagram of the particulate alkali metal niobatehaving a rectangular cuboid shape. In FIG. 1, the length of a side alongthe x-direction is represented as L1, the length of a side along they-direction is represented as L2, and the length of a side along thez-direction is represented as L3. Here, the lengths satisfy therelation: L1<L3<L2. Thus, the L_(max) corresponds to L2, and the L_(min)corresponds to L1 in FIG. 1. Referring to FIG. 1, the particle of thepresent invention has the length L2 of within the range of 0.10 to 25μm, and the length L1 of within the range of 0.050 to 15 μm.

The L_(max) is preferably 0.10 to 20 μm.

The L_(min) is preferably 0.050 to 10 μm, more preferably 0.050 to 4 μm,and further preferably 0.050 to 2 μm. Furthermore, it is particularlypreferably 0.050 to 1.5 μm.

In particular, in the case where the alkali metal niobate is potassiumniobate, particles with smaller size can be produced. Specifically,rectangular cuboid particles with the length of each side of withinabout 0.050 to 1.5 μm can be obtained.

Any method can be used to determine the lengths of sides of the particleand the L_(max) and L_(min) of the substantially rectangular cuboidparticles, and the method is not particularly limited. The lengths ofsides, L_(max), and L_(min) can be determined, for example, in thefollowing manner: Firstly, a microphotographic image of the particulatealkali metal niobate is taken using a scanning electron microscopy(SEM), and then the lengths of sides of each particle are read out fromthe image.

Also, it is one of the characteristics of the present invention thatvariation of particle sizes is small over the whole of powdery alkalimetal niobate (which is the same meaning as particulate alkali metalniobate). Preferably, at least 80% of the total rectangular cuboidparticles in the powder have a length of each side of the rectangularcuboid of within the range of 0.050 to 25 μm. The ratio is morepreferably at least 90%, and still more preferably at least 95%.

In a preferred embodiment of the present invention, a ratioL_(max)/L_(min) (the ratio of the length of the longest side to thelength of the shortest side in the substantially rectangular cuboid) iswithin the range of 1 to 5.

Referring to FIG. 1, L_(max) corresponds to L2, and L_(min) correspondsto L1. Thus, the feature “the ratio L_(max)/L_(min) is within the rangeof 1 to 5” has the same meaning as that the ratio L2/L1 is within therange of 1 to 5 in FIG. 1.

The ratio L_(max)/L_(min) is preferably 1 to 3, more preferably 1 to 2,still more preferably 1 to 1.5, and particularly preferably 1. A statethat satisfies the relation, “the ratio L_(max)/L_(min) is 1”, meansthat the shape of a particle is cubic.

The method of preparing particulate alkali metal niobate is notparticularly limited. The method described above, which is a firstaspect of the present invention, is preferable as the method ofpreparing the particulate alkali metal niobate. The method is innovativein that the particle size can be controlled simply by a chemicalprocess, and no physical processes such as grinding are necessary. Thus,the method is advantageous in that production processes can besimplified as compared to conventional methods. In addition, the methodaccording to a first aspect of the present invention can control thesize of particles, and prevent agglomeration of the particles, while itis generally difficult to control variation in particle size in physicalgrinding or the like conventional method. As a result, particles withhighly-controlled size can be obtained by the method according to afirst aspect of the present invention. Because of these reasons, themethod of a first aspect of the present invention is preferable as amethod for preparing particulate alkali metal niobate.

<Piezoelectric Ceramic Materials>

A third aspect of the present invention relates to a piezoelectricceramic material including the particulate alkali metal niobate.

A method to produce the piezoelectric ceramic material is notparticularly limited. Generally, the piezoelectric ceramic material maybe produced by mixing dried particulate alkali metal niobate withrequired additives such as an organic binder, a dispersant, aplasticizer, and a solvent, to prepare a composition. Then, an articleis molded from the composition through a known molding method, and thearticle is sintered at a high temperature (about 1,000° C.). Examples ofsuch a known molding method include press molding or molding using amold.

Then, by forming electrodes on a molded body obtained from thepiezoelectric ceramic material, piezoelectric elements such as apiezoelectric buzzer and a piezoelectric transducer can be produced.

EXAMPLES

The present invention will be described in more detail based on thefollowing examples. It is to be noted that the present invention is notlimited to these examples. In the examples and comparative examplesbelow, the unit “M”, which is used to refer to a concentration of analkali or acid solution, means mol/L unless otherwise indicated.

Example 1 (Synthesis 1 of Substantially Cubic NaNbO₃ Particles)

A portion of 27.02 g (=100 mmol) of niobium chloride was completelydissolved in 150 mL of a 0.10-M aqueous HCl solution. The solution wastransferred into a 200-mL volumetric flask, and a 0.10-M aqueous HClsolution was further added to the flask for adjusting the total volumeof the solution to be 200 mL. A 0.50-M aqueous NbCl₅ solution in 0.10 MHCl was thus prepared. Then, 6.0 mL of the 0.50-M aqueous NbCl₅ solutionin 0.10 M HCl was slowly added to 6.0 mL of a 12.0-M aqueous NaOHsolution in a 30-mL Teflon(™) vessel at room temperature under stirring.The resulting white suspension was allowed to still stand with heatingat 100° C. in the Teflon(™) vessel for 24 hours. Then, the content wastransferred to an autoclave whose inner chamber wall was made ofTeflon(™), and allowed to stand for 3 hours with heating at 250° C. Thesolid matter was separated from the resulting suspension bycentrifugation, and then the solid was dispersed in water underultrasonic dispersing. The solid matter was separated again bycentrifugation and dried to separate particulate sodium niobate. Thesize and shape of the obtained solid particles were observed by ascanning electron microscope (SEM, manufactured by HITACHI, Ltd.,S-4800), and the crystal structure of the solid particles was evaluatedby X-ray diffraction (XRD, manufactured by Rigaku Corporation,Ultima-IV, 40 kV, 40 mA). FIGS. 2 and 3 show a SEM image and an XRDpattern of the obtained NaNbO₃ particles, respectively. Results of theevaluation found that the particles were cubic particles having thelength of one side of about 2 μm, and that particles consisted of singlephase NaNbO₃. Alternatively, the particles can be obtained when thetemperature during the second heating step is set to 200° C. Byadjusting the initial NaOH concentration to 2 to 18 mol/L, the particlesize may be controlled such that the length of the longest side shouldfall within the range of 0.10 to 25 μm, and the length of the shortestside should fall within the range of 0.050 to 15 μm.

Example 2 (Synthesis 1 of Substantially Cubic KNbO₃ Particles)

Synthesis of substantially cubic KNbO₃ particles was performed in thesame manner as in Example 1, except that an 18.0-M aqueous KOH solutionwas used in place of the 12.0-M aqueous NaOH solution. FIGS. 4 and 5show the SEM images of the obtained KNbO₃ particles. As shown in the SEMimages, cubic particles having sides of about 1 μm was obtained. Also,XRD analysis of the cubic particle was carried out, and the analysisfound that particles consisted of single phase KNbO₃. Alternatively,when the temperature during the second heating step (step (C)) was setto 150° C., cubic KNbO₃ particles with a narrow particle sizedistribution of about 0.2 μm, as shown in FIG. 6, were obtained,although product yield was slightly lowered.

Example 3 (Synthesis 2 of Substantially Cubic NaNbO₃ Particles)

A portion of 6.0 mL of an 8.0-M aqueous NaOH solution was added to 0.40g (=3.0 mmol) of niobium pentoxide in a 30-mL Teflon(™) vessel.Ion-exchange water was further added to the vessel under stirring toadjust the total volume of the mixture to 12 mL. Then, the Teflon(™)vessel was sealed and allowed to still stand with heating at 100° C. for24 hours . Then, the content was transferred to an autoclave whose innerchamber wall was made of Teflon(™), and allowed to stand for 3 hourswith heating at 250° C. The solid matter was separated from theresulting suspension by centrifugation, and then the solid was dispersedin water under ultrasonic dispersing. The solid matter was separatedagain by centrifugation and dried to separate NaNbO₃ particles.Evaluation of the obtained particles was performed in the same manner asthat described in Example 1. By adjusting the initial concentration ofNaOH to 5 to 18 mol/L, the size of the substantially rectangular cuboidparticles may be controlled such that the lengths of the sides shouldfall within the range of 0.50 to 25 μm.

Example 4 (Synthesis 2 of Substantially Cubic KNbO₃ Particles)

Synthesis of substantially cubic KNbO₃ particles using niobium pentoxideas a starting material was performed in the same heating procedures andwashing operation as those in Example 3, except that a KOH solution orparticulate KOH was added in place of the aqueous NaOH solution tocontrol the alkaline ion concentration to 18 M. FIG. 7 shows a SEM imageof the obtained KNbO₃ particles. The image shows that the particles hada cuboid shape with the lengths of the sides of about 0.5 μm. Moreover,the XRD analysis of the obtained particles found that the articles wereKNbO₃ particles having an orthorhombic crystal structure.

Example 5 (Synthesis 3 of Substantially Cubic NaNbO₃ Particles)

A portion of 6.0 mL of the 0.50-M aqueous NbCl₅ solution in 0.10 M HClwas added to 6.0 mL of a 8.0-M NaOH solution under stirring to prepare awhite suspension. The white suspension was allowed to still stand withheating at 100° C. for 24 hours in a Teflon(™) vessel. The content wastransferred to an autoclave whose inner chamber wall was made ofTeflon(™), and allowed to stand for 3 hours with heating at 250° C.Then, the solid matter was separated from the resulting suspension bycentrifugation, and then the solid was dispersed in water underultrasonic dispersing. The solid matter was separated again bycentrifugation and dried to separate particulate sodium niobate. Thesize and shape of the obtained solid particles were observed by ascanning electron microscope, and the crystal structure of the solidparticles was evaluated by X-ray diffraction. FIG. 8 shows the SEM imageof the obtained NaNbO₃ particles.

Comparative Example 1

(Synthesis of Substantially Cubic NaNbO₃ Particles, in which the FirstHeating Step was Omitted)

Particulate sodium niobate was obtained in the same manner as in Example5, except that the step of allowing to still stand with heating at 100°C. for 24 hours was omitted. The size and shape of the obtained solidparticles were observed by a scanning electron microscope, and thecrystal structure of the solid particles was evaluated by X-raydiffraction. FIG. 9 shows the SEM image of the obtained particles.

Comparison of Example 5 with Comparative Example 1 found that thepre-heating treatment at 100° C. contributed to produce uniform-sizesubstantially cubic NaNbO₃ particles while preventing agglomeration.According to the result of the XRD analysis, both of the particles shownin FIGS. 8 and 9 consisted of single phase NaNbO₃.

Example 6 (Synthesis of Substantially Cubic KNbO₃ Particles)

To an alkaline solution (36 M KOH solution, 185 mL) put in a Teflon(™)vessel, 185 mL of an dispersion containing 12.3 g of Nb₂O₅ was addeddropwise at a rate of 15 mL/min under stirring. The thus-obtained mixedsuspension was stirred for 10 minutes in a Teflon(™) vessel. Theresultant suspension was transferred to an autoclave whose inner chamberwall was made of Teflon(™). The suspension was heated up to 100° C. over30 minutes under stirring, and was then stirred at 100° C. for 24 hours.Next, the suspension was heated up to 200° C. over two and a half hours,followed by heating at 200° C. for three hours under stirring. After theheating, the suspension was naturally cooled. The solid matter wasseparated from the resulting suspension by centrifugation. Then, thesolid matter was dispersed in water under ultrasonic dispersing,separated by centrifugal segmentation, and decantation. This washingprocess including ultrasonic dispersing, centrifugal segmentation, anddecantation was repeated six times. Washing by centrifugation wasfurther made three times using acetone as a washing liquid, followed bydrying in a desiccator. Thereby, particulate sodium niobate wasobtained. The size and shape of the obtained solid particles wereobserved by a scanning electron microscope, and the crystal structure ofthe solid particles was evaluated by X-ray diffraction. FIGS. 10 and 11show the SEM image of the synthesized particles. FIG. 12 shows theresult of the XRD analysis. The particles were in a cubic shape. Thediffraction pattern indicated that the obtained particles consisted ofKNbO₃ rhombohedral crystal.

Example 7 (Preparation of KNbO₃ Ceramics by Sintering and Evaluation ofPiezoelectric Properties)

The KNbO₃ particles prepared in Example 2 were pelletized and thensintered at various different sintering temperatures. The piezoelectricproperties of the obtained ceramics were evaluated. Table 1 shows valuesof the properties.

In Table 1, the “sintering temp.” refers to sintering temperatures, the“ρ” refers to a sintering density, which was calculated based on thesize (volume) and weight of the particles. The “tan δ” refers to adielectric loss measured with an impedance analyzer. Further, the “ε₃₃^(T)/ε₀” refers to a dielectric constant measured with an impedanceanalyzer. The “Kp” refers to an electromechanical coupling coefficient,which was calculated based on values of resonance frequency andantiresonance frequency measured with an impedance analyzer. The “Np”refers to a frequency constant calculated based on a value of resonancefrequency measured with an impedance analyzer and element diameter, andthe “d33” refers to a piezoelectric constant measured with a d33 meter.

TABLE 1 Sintering Temp. ρ tan δ K_(P) N_(P) d33 No. ° C. g/cm³ % ε₃₃^(T)/ε₀ % Hz · mm pC/N 1 1020 4.05 8.72 877 25.5 3220 133.4 2 1040 4.076.77 875 27.2 3297 120 3 1060 3.76 8.68 727 21.4 3000 83.7

As shown in Table 1, the KNBO₃ ceramics obtained in the presentinvention showed high piezoelectric properties. Especially in the casewhere the sintering temperature was set to 1,020° C., an highpiezoelectric property as d33 of 133.4 was achieved.

INDUSTRIAL APPLICABILITY

The production method of the present invention is a method to providefine particulate alkali metal niobate directly and simply by a chemicalprocess, which does not need any physical processes such as grinding.According to the method of the present invention, substantiallyrectangular cuboid particles can be provided by a liquid phase.Agglomeration of the particles can be prevented due to the substantiallyrectangular cuboid shape, and thereby fine particles with smallvariation in particle sizes can be produced. The particles thus obtainedare micrometer-order-size particles with easy handleability, and suchparticles can be suitably used as a piezoelectric material.

EXPLANATION OF SYMBOLS

-   L1: Length of a side along the x-direction-   L2: Length of a side along the y-direction-   L3: Length of a side along the z-direction

1. A method of producing particulate alkali metal niobate represented byMNbO₃   (1) wherein M represents one element selected from alkalinemetals, the particulate alkali metal niobate having a substantiallyrectangular cuboid shape, the method comprising the steps of: (a) mixinga niobium-containing solution with an alkaline solution having aconcentration of 0.1 to 30 mol/L, to prepare a suspension; (b) stillstanding the suspension at between 80° C. and 150° C. for 12 to 48hours; (c) performing solvothermal reaction of the suspension at between150° C. and 300° C. for 1 to 12 hours; and (d) separating theparticulate alkali metal niobate from a reaction mixture.
 2. The methodaccording to claim 1, wherein M in the formula (1) is Na, and thealkaline solution is NaOH.
 3. The method according to claim 1, wherein Min the formula (1) is K, and the alkaline solution is KOH.
 4. The methodaccording to claim 1, wherein the niobium-containing solution includes:niobium oxide and/or niobium halide; a solvent selected from the groupconsisting of water, ethylene glycol, and polyethylene glycol; and anacid.
 5. Particulate alkali metal niobate represented by the formula(1):MNbO₃   (1) wherein M represents one element selected from alkalinemetals, the particulate alkali metal niobate having a substantiallyrectangular cuboid shape, wherein the substantially rectangular cuboidshape has a longest side and a shortest side, the length of the longestside represented by an index L_(max) is 0.10 to 25 μm, and the length ofthe shortest side represented by an index L_(min) is 0.050 to 15 μm. 6.The particulate alkali metal niobate according to claim 5, wherein theratio of the L_(max) to the L_(min), L_(max)/L_(min), is within therange of 1 to
 5. 7. The particulate alkali metal niobate according toclaim 5, wherein the M in the formula (1) is Na or K.
 8. A particulatealkali metal niobate represented by the formula (1):MNbO₃   (1) wherein M represents one element selected from alkalinemetals, the particulate alkali metal niobate having a substantiallyrectangular cuboid shape, wherein the substantially rectangular cuboidshape has a longest side and a shortest side, the length of the longestside represented by an index L_(max) is 0.10 to 25 μm, and the length ofthe shortest side respresented b an index L_(min) is 0.050 to 15 μm,wherein the particulate alkali metal niobate is prepared by the methodaccording to claim
 1. 9. A piezoelectric ceramic material comprising theparticulate alkali metal niobate according to claim 5.